In earthquake monitoring and the vibration monitoring of moving automobiles, ships and aircrafts, theoretically it is impossible to obtain a reference point of the inertial frame; and in the vibration test of foundation and architectural pile foundation, and the vibration monitoring of huge equipment with vibration source, such as turbines, steamturbines, generators, etc., it is difficult to find a reference point of the inertial frame on the surface of the area nearby, because the vibration of the equipment itself leads to the vibration of the foundation, so the vibration signal measured is obviously not accurate or mixed with the vibration interference in the reference point. For instance, conventional eddy-currrent amplitude sensor and transformer type sensor are all relative measuring sensors. Adopting the former sensor, Bently Company of U.S.A. installed another set of velocity meters at the reference point to measure the vibration of the reference point itself, and then counteracted the interference by the inverse-phase superimposition of the two sets of signals; but the realization of this method is complicated and quite difficult (Introduction of products of Bently Company in 1989). In case of the latter sensor, it is necessary to fix the primary or secondary winding on an motionless body near the vibrating body measured, but the distance between the probe and the surface of the vibrating body measured is so short that no motionless body can exist within such a short distance, therefore in the practice of its application, a probe is installed on a body with relatively weak vibration as the reference point, and the result measured thereby is obviously not accurate enough.
In addition, although the Chinese Patent No. 87105424, namely, "Suspended Type Eddy-Current Earthquake Wave Detector" has the advantages that the natural frequency of the vibrating system is adjustable and that the transverse effect is weakened, with this apparatus, the magnitude of the eddy-current magnetic field is a function of the relative movement velocity between the main magnet and the eddy-current sleeve, and the inductive electromotive force in the coil is a function of the velocity of the coil in the eddy-current magnetic field, in this way a "two-stage effect" is produced, that is to say that the electric signal produced in the fixed wave detector coil is proportional to the acceleration of the movement of the outer casing. Furthermore, the Chinese Patent No. 92102774.5, namely, "Apparatus for Measuring the Earthquake Type Low Frequency Vibration", adopts a kind of commonly used earthquake type vibration velocity sensor, however, in order to extract the amplitude, it is necessary to resort to the revision of multiple integral or integral circuit or of the computer, and it is difficult to extract the accurate amplitude data.